1. Field of the Invention
The present invention relates to a method and system for treating a semiconductor material. More specifically, the present invention relates to a method and system for preheating a semiconductor material and reducing the amount of laser radiation required to achieve downstream surface melting on at least one side while also enabling controlled recrystallization and cooling.
2. Description of the Related Art
The ion implantation process, which introduces impurity atoms or dopants into surface region of a semiconductor wafer leaves dopant atoms in interstitial sites where they electronically inactive. In order to move the dopant atoms into substitutional sites in the lattice to render them active and otherwise to repair process damage an annealing of the surface region is performed by heating to high temperature, typically in a tube furnace or with a rapid thermal process (RPT) furnace.
The absorption depth of a given wavelength of light in a material decreases as temperature of the material increases. An example is the absorption of silicon as a function of temperature as is shown in FIG. 1, provided by Thomas R. Harris, “Optical Properties of Si, Ge, GaAs, InAs, and InP at elevated Temperatures, Thesis AFIT/GAP/ENP/10-M08, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio, 2010 the contents of which are incorporated by reference, Also for review by the public is U.S. Pat. No. 7,494,942 and U.S. Pat. No. 7,399,945, the contents of which are also incorporated by reference.
Necessarily, for the purpose of annealing ion implanted semiconductor wafers having a crystal lattice structure using an JR laser, much of the laser radiation is used to heat the wafer to a point where most of the laser radiation is absorbed close to the surface of the wafer facing the incident beam (incident side). Ultimately the material reaches a temperature where most of the radiation is absorbed near the surface and a thin layer of the material near the surface melts further changing the absorption rate therein.
Conventionally, an incident laser beam impinges on only a tiny part of a wafer on an incident side at any moment in time resulting in substantial localized thermal gradients, localized large stress gradients, and wafer fracture.
The amount of incident laser radiation required to achieve surface melting can be significantly reduced if the wafer is preheated, prior to heating the surface to a higher desired melting temperature. There are several methods available for wafer preheating, including; conductive source heating via resistance, conduction from a susceptor heated by RF (Radio Frequency), and radiative heating by JR (Infra Red) light source (non-laser).
The process using ion implantation to generate a semiconductor junction provides for a two-step process; a first step of “ion implantation” at a specific ion energy and dose (so-called pre-deposition) and a second step of “annealing” (also drive-in diffusion). The later is performed in two ways: (A) heating of an ion-implanted wafer in a furnace to a temperature of >1000° C. for a time period of >0.50 hr (allowing implanted species to migrate), and (B) rapidly heating a surface of an implanted waver with a heat source (allowing rapid migration to active sites), often in a process called rapid thermal processing (RTP)).
In a process referred to as GILD (Gaseous Immersion Laser Doping), laser heating (surface melting) by laser energy when performed in an appropriate gaseous environment containing a desired doping species (including but not limited to Arsine (AsH3), Phosphine (PH3), and/or Boron Triflouride (BF3) or others as is known in the art) was found to result in high quality semiconductor junctions and eliminated ion implantation and lowering capital equipment cost substantially. It was essential to the GILD process to employ short pulsed and short wavelengths lasers operating in the UV spectrum (Excimers). This was essential due to the short absorption depth of UV radiation in silicon.
Accordingly, there is a need for an improved method and system for preheating of semiconductor materials for laser annealing and gas immersion laser doping so that the amount of laser radiation required to achieve further processing is significantly reduced with enhanced processing economics. There is also a need to eliminate material fracture arising during localized heating by a scanned laser beam during processing.